This invention relates to an optical branching element which is used for an optical communication system, an optical measuring system or the like.
A bilateral optical communication transmission line has been used which is composed of one central information processing unit and a plurality of terminal units which are in remote places and are connected to the central information processing unit by a trunk optical fiber and branch optical fibers which are branched from the trunk optical fiber, such that different pieces of information can be transmitted from the plurality of terminal units to the central information processing unit and conversely a common piece of information can be transmitted from the central information processing unit to each terminal unit. In this optical communication transmission line, it is necessary to optically connect the trunk optical fiber and branch optical fibers which are connected to each terminal unit with optical branching/joining elements.
Heretofore as an optical branching/joining element for use in such a case, that which utilizes a reflection film such as a half-mirror and that which utilizes coupling between optical fibers have been proposed. That which utilizes a reflection film has an advantage in that the branching ratio of a transmitted optical signal can be varied over a wide range by selecting a characteristic of the reflection film, but branching is limited to one direction with respect to a single reflection film. Therefore, in order to provide branching in a plurality of directions, such as two or three, there needs to be two or three reflection films.
In the construction of such a transmission line which uses a reflection film, optical transmission characteristics largely depend on the location of the optical fiber, the SELFOC lens and the reflection film. Therefore, many stages of a delicate locating process are required in the construction of this kind of transmission line which uses reflection films.
On the other hand, in the type which utilizes coupling between optical fibers it is necessary to remove the coatings of the optical fibers to be coupled, and after a minute processing of the coupling portions of the optical fibers with the object of tapering them, the optical fibers are optically connected in such a manner as to join them at the tapered portions. This type which utilizes coupling has the following drawbacks: the preliminary minute processing necessary for joining them is delicate, whereby it is difficult to obtain a secured product; and it is susceptible to external influences such as heat and vibration.